diff --git a/README.md b/README.md
new file mode 100644
index 0000000..8eebf63
--- /dev/null
+++ b/README.md
@@ -0,0 +1,3 @@
+A small webapp to read the sensors provided on modern mobile devices.
+
+On Android use the Firefox browser.
diff --git a/sources/ambientLight.js b/sources/ambientLight.js
index 6c86f1e..bd1e9d4 100644
--- a/sources/ambientLight.js
+++ b/sources/ambientLight.js
@@ -26,7 +26,7 @@ var ambientLightSource = (function(window) {
         lastVal = e.value;
 
       if (eventCounter == 5) {
-        var percentDiff = percentDifference(lastVal, e.value);
+        var percentDiff = Math.abs(percentDifference(lastVal, e.value));
 
         //console.log('devicelight', lastVal, e.value, percentDiff)